Modern automotive engines contain electronic engine control systems which vary operating parameters of the engine, such as air-fuel ratios and ignition timing, to achieve optimum performance. Such control systems are capable of changing engine operating parameters in response to a variety of external conditions.
A primary function of electronic engine control systems is to maintain the ratio of air and fuel at or near stoichiometry. Electronic engine control systems operate in a variety of modes depending on engine conditions, such as starting, rapid acceleration, sudden deceleration, and idle. One mode of operation is known as closed-loop control. Under closed-loop control, the amount of fuel delivered is determined primarily by the concentration of oxygen in the exhaust gas. The concentration of oxygen in the exhaust gas being indicative of the ratio of air and fuel that has been ignited.
The oxygen in the exhaust gas is sensed by a Heated Exhaust Gas Oxygen (HEGO) sensor. The electronic fuel control system adjusts the amount of fuel being delivered in response to the output of the HEGO sensor. A sensor output indicating a rich air/fuel mixture (an air/fuel mixture above stoichiometry) will result in a decrease in the amount of fuel being delivered. A sensor output indicating a lean air/fuel mixture (an air/fuel mixture below stoichiometry) will result in an increase in the amount of fuel being delivered.
Engines which are capable of operating on different fuels, such as gasoline, methanol, a mixture of the two, or natural gas, utilize electronic engine control systems to change the engine operating parameters in response to the type of fuel being delivered to the engine. Such systems utilize a sensor to detect the type of fuel being delivered to the engine and an electronic engine control to vary the operating parameters accordingly. An instance of such a system is disclosed in Wineland et al. in U.S. Pat. No. 4,706,629.
Unfortunately, it is possible for the sensor to indicate erroneous mixture values under certain operating conditions. For example, in the instance of liquid fuels, if the fuel is 100% gasoline, but of a different composition than that used to "calibrate" the sensor, the sensor might indicate that the fuel is a mixture of gasoline and methanol rather than 100% gasoline. Since different mixtures of gasoline and methanol all have different burn rates and stoichiometric air/fuel values, errors in sensing the mixture can produce poor driveability, bad air/fuel control, and excessive exhaust emissions.
A similar problem occurs with engines that use natural gas as a fuel. Natural gas is an unregulated fuel which consists mostly of methane with varying amounts of ethane, propane, butane, and other inert gases. Each of these gases are characterized by different combustion properties, and consequently, the air/fuel ratio and ignition timing requirements differ with the gas composition. As with liquid fuels, errors in sensing the mixture can produce poor driveability, bad air/fuel control, and excessive exhaust emissions.
In the event that the flexible fuel sensor fails, the operating parameters which are dependent on the output of the fuel type sensor will be erroneous. Unlike systems which operate with a single type of fuel, an engine capable of, and actually operating on different fuels cannot revert to a preset value if the fuel sensor fails. As a result, the electronic fuel control system will continually vary the operating parameters to adjust for a detected fuel which is erroneous. Such a condition will at best lead to poor driveability, bad air/fuel control, and excessive exhaust emissions, and at worst can lead to complete failure of the engine.